1. Field of the Invention
This invention relates to improving the performance of a high temperature fuel cell power plant, and, in particular, to enhancing the stability of a combustion reaction in a mixer/eductor/oxidizer system when mixing anode-outlet or exhaust gas with inlet air or oxidant gas for supply to the stack cathode side.
2. Description of the Related Art
A fuel cell is a device that directly converts chemical energy stored in any hydrogen or hydrocarbon containing fuel such as hydrogen, methane, or natural gas into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional processes in both increased efficiency and reduced pollutant emissions.
In general, a fuel cell, similar to a battery, includes a negative or anode electrode and a positive or cathode electrode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell and its neighbor.
In high temperature fuel cell stacks, fresh air usually serves as oxidant and is provided at the entry of the cathode-side of the stack. This fresh air is typically at ambient temperature and must be heated to the operating temperature of the stack. Conventionally, unused fuel in the exhaust gas exiting from the anode-side of the stack and incoming fresh air are burned to heat the air. In order to ensure complete reaction of fuel and to minimize temperature gradients, the anode-exhaust must be completely mixed with air.
It is also desirable to minimize the pressure differential of the gases passing through the anode and cathode sides of the stacks in order to provide and maintain seals to keep the fuel and oxidant gases isolated from each other. To create the required seals, surfaces, which in some cases sandwich a gasket, are mechanically forced together to realize an “acceptable” leak rate. This leak rate is a function of the pressure differential. Therefore, minimizing the pressure differential is important to prevent excessive leaks.
During operation of the fuel cell stack, at a junction of the two process gas streams, gas pressure at the exit of the anode-side of the stack is coupled to gas pressure at the inlet of the cathode-side of the stack. Typically, the pressure at the exit of the anode-side is necessarily higher than the pressure at the inlet of the cathode-side by an amount required to overcome pressure losses associated with any connection piping and with the oxidizer used to burn the anode exhaust and incoming oxidant gases.
A mixer/eductor/oxidizer (MEO) system has been introduced into molten carbonate fuel cells to address both the gas mixing and the pressure differential problems. The MEO system oxidizes unconverted anode fuel, preheats inlet air, recycles carbon dioxide (CO2) to the cathode, and reduces the pressure difference between the anode and cathode gas streams. However, the unconverted anode fuel BTU levels vary at different operating power levels of the fuel cell stack. Maintaining required outlet temperatures to accommodate cathode side operational requirements and oxidizer bed catalyst temperatures requires adjusting the amount of additional air to be provided to the fuel cell. Swings in ambient temperatures and blower capacity limitations impact the delicate balance of many parameters required for desired fuel cell stack operation.
Applicants found that igniting and burning anode exhaust and incoming air, while mixing, prior to the mixture passing through the catalyzed oxidizer bed, would improve the fuel cell system performance. However, maintaining this “pre-ignition” state in a conventional mixer/eductor/oxidizer for all operating conditions of the fuel cell stack has been difficult. Specifically, once a fuel system has been designed and built, operational changes in the system are the primary means to establish and maintain pre-ignition in the eductor. To enhance the stability of pre-ignition, the following operational adjustments may be made: (i) reducing fuel utilization to increase the BTU content of the unspent fuel, (ii) reducing the airflow to make combustion less lean and to reduce air inlet velocity, and/or (iii) raising the overall temperatures to improve reactivity. However, all these operational solutions have deleterious consequences to the overall operation of the fuel cell stack.
In particular, reduction in the fuel utilization negatively affects the overall system efficiency (power produced/fuel consumed), and further affects the amount of cooling air required and the overall fuel cell stack temperatures. Reduction in the airflow usually increases power consumption by the blower. This is because the fuel cell stack continues to require the same amount of air to maintain the overall temperature and diverting some of the airflow away from the eductor results in higher pressure drop paths for injecting the air to other parts of the fuel cell system. The increase in the power consumption by the blower reduces the total power produced by the fuel cell stack. Raising fuel cell operating temperature impacts the fuel cell stack performance and its operating life, and increases heat losses from the fuel cell module.
Addressing the “pre-ignition” state problem using conventional equipment designs has also been impractical. This is because MEOs in typical fuel cell systems run very fuel lean. The typical operating range of MEOs has an air/oxidant to fuel ratio between 500% and 700% of a stoichiometric ratio, which results in very low temperatures and unstable flames. For example, U.S. Pat. No. 6,902,840, assigned to the same assignee hereof, discloses a basic mixer/eductor system for the high temperature fuel cells, which is adapted to provide both the desired mixing of oxidant gas or air and the anode exhaust gas and a reduced pressure difference between the gas at the cathode-side inlet and the gas at the anode-side outlet of the fuel cell stack. International application PCT/US2004/037889, also assigned to the same assignee hereof, discloses an improved mixer/eductor, the performance of which is improved by refined eductor nozzle configurations. Although the '840 patent highlighted the benefits of achieving a pre-ignition state in the mixer/eductor, a stable and continuous pre-ignition state using the mixer/eductor configurations of the '840 patent and of the '889 application has been difficult to achieve.